It is further known to restrict the interference noises at least partially by activating or evaluating the noise measurement only within a fixed measuring window which is started for example after the injection of a first injection amount and terminated on completion of combustion of a following injection pulse. Although this method brings a certain improvement as regards the evaluation of the detected combustion noise, it still includes a high proportion of undesirable interference noises. Moreover, the fixed measuring window cannot be adjusted to the individual injection pulses with their different injection amounts. However, since the intensity of the combustion noise is a measure for the amount of fuel injected, the interference noises contained in the recorded combustion noise can lead to unreliable results in the evaluation. The known method is therefore to be regarded as critical, for example, for determining an injected amount of fuel.